Adaptive modulation for fixed wireless link in cable transmission system

ABSTRACT

In one embodiment of a communications network, the predetermined encoding scheme and symbol constellation configurations are chosen so that the range in channel qualities that the encoding scheme and symbol constellation configurations are designed to be utilized within overlap with each other. This overlapping provides hysteresis, which reduces the frequency with which a subscriber must alter encoding scheme and symbol constellations. Reducing the frequency of changing encoding scheme and/or symbol constellation eliminates the communication overhead associated with these changes and increases throughput by enabling the subscriber to spend more time transmitting data.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/261,234,filed Oct. 27, 2005, which is a continuation of application Ser. No.09/858,926, filed May 15, 2001, which claimed the benefit of provisionalApplication No. 60/241,046, filed Oct. 16, 2000, the disclosures ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to communication systems and morespecifically to adaptive modulation in a fixed wireless communicationsystem.

BACKGROUND

The growth of digital communications has created an increased demand forbroadband communications infrastructure. A significant component of thecost of creating this infrastructure is the cost of providing fixedcabling in the Customer Access Network (“CAN”). A method of reducing thecost of a broadband CAN is to use fixed broadband wireless to providecommunication links between subscribers and a fixed backbone network.

The Broadband Wireless Internet Forum (“BWIF”) has created a standardfor the provision of fixed broadband wireless. The standard involves theuse of a Wireless Access Termination System (“WATS”) to broadcastinformation to a group of subscribers on a downstream channel. Thesubscribers send information to the WATS using a shared upstreamchannel. Each subscriber is allocated access to a channel in accordancewith a Medium Access Protocol (“MAC”).

A limitation of the BWIF specification is that the subscriberscommunicate on the upstream channel using the same modulation andencoding schemes. The characteristics of the upstream channel can varydepending on the location of an individual subscriber. Therefore, use ofa modulation and encoding scheme that is appropriate for certain channelconditions can unnecessarily impact on the Quality of Service (“QoS”)experienced by individual subscribers. If a modulation scheme is chosento provide high throughput in a channel with high Signal to Interferenceplus Noise Ratio (“SINR”), then subscribers in locations where theupstream channel has a low SINR will experience very high Bit ErrorRates (“BER”). Alternatively, if a modulation scheme is chosen toprovide low BERs in a channel with a low SINR, then subscribers inlocations where the upstream channel has a high SINR can experiencesub-optimal throughput. This problem can be overcome by allowingindividual subscribers to transmit using a modulation scheme thatcontinually adapts to the channel conditions experienced by thesubscriber in order to provide near optimal throughput for those channelconditions.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The present invention is described with reference to the accompanyingdrawings. In the drawings, like reference numbers indicate identical orfunctionally similar elements. Additionally, the left most digit(s) of areference number identifies the drawing in which the reference numberfirst appears.

FIG. 1 is a semi-schematic view illustrating a wireless communicationsnetwork;

FIG. 2 is a semi-schematic view illustrating a wireless accesstermination system connected to a backbone network and a wireless modemconnected to customer premise equipment at a subscriber location;

FIG. 3 is a flow diagram illustrating steps performed by a wirelessmodem to transmit data on an upstream channel;

FIG. 4 is a semi-schematic circuit diagram of a wireless modem;

FIGS. 5A and 5B are flow diagrams illustrating steps performed by awireless access termination system to decide which encoding scheme andsymbol constellation a subscriber should use to transmit on an upstreamchannel;

FIG. 6 is a graph that generally illustrates different characteristicsof an additive white Gaussian noise channel, a Ricean channel and aReyleigh fading channel;

FIG. 7 is a semi-schematic block diagram illustrating components of awireless access termination system used to determine which encodingscheme and symbol constellation a subscriber should use to transmit onan upstream channel;

FIG. 8 is a flow diagram illustrating steps used by a wireless accesstermination system to allocate an encoding scheme and symbolconstellation to a subscriber registering with the wireless accesstermination system;

FIG. 9 is a flow diagram illustrating communications between a wirelessmodem and a wireless access termination system that cause the wirelessmodem to adopt a new encoding scheme and/or symbol constellation;

FIG. 10 is a timing diagram illustrating a delay between a decision by awireless access termination system that a wireless modem should changeencoding scheme and/or symbol constellation and the wireless modemchanging encoding scheme and/or symbol constellation;

FIG. 11 is a flow diagram illustrating steps performed by a wirelessaccess termination system to allocate transmission timeslots on anupstream channel to subscribers;

FIG. 12 is a semi-schematic block diagram illustrating components of awireless access termination system used to determine which encodingscheme and symbol constellation a subscriber should use to transmit onan upstream channel and to allocate timeslots on the upstream channel tosubscribers; and

FIG. 13 is a flow diagram illustrating the steps performed by a wirelessaccess termination system to allocate timeslots to subscribers, when asubscriber changes encoding scheme and/or symbol constellation.

The present invention will now be described with reference to theaccompanying drawings. In the drawings, like reference numbers generallyindicate identical, functionally similar, and/or structurally similarelements. The drawing in which an element first appears is indicated bythe leftmost digit(s) in the reference number.

DETAILED DESCRIPTION OF THE INVENTION

The following Detailed Description refers to accompanying drawings toillustrate exemplary embodiments consistent with the invention.References in the Detailed Description to “one exemplary embodiment,”“an exemplary embodiment,” “an example exemplary embodiment,” etc.,indicate that the exemplary embodiment described may include aparticular feature, structure, or characteristic, but every exemplaryembodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same exemplary embodiment. Further, when a particularfeature, structure, or characteristic is described in connection with anexemplary embodiment, it is within the knowledge of those skilled in therelevant art(s) to effect such feature, structure, or characteristic inconnection with other exemplary embodiments whether or not explicitlydescribed.

The exemplary embodiments described herein are provided for illustrativepurposes, and are not limiting. Other exemplary embodiments arepossible, and modifications may be made to the exemplary embodimentswithin the spirit and scope of the invention. Therefore, the DetailedDescription is not meant to limit the invention. Rather, the scope ofthe invention is defined only in accordance with the following claimsand their equivalents.

The following Detailed Description of the exemplary embodiments will sofully reveal the general nature of the invention that others can, byapplying knowledge of those skilled in relevant art(s), readily modifyand/or adapt for various applications such exemplary embodiments,without undue experimentation, without departing from the spirit andscope of the present invention. Therefore, such adaptations andmodifications are intended to be within the meaning and plurality ofequivalents of the exemplary embodiments based upon the teaching andguidance presented herein. It is to be understood that the phraseologyor terminology herein is for the purpose of description and not oflimitation, such that the terminology or phraseology of the presentspecification is to be interpreted by those skilled in relevant art(s)in light of the teachings herein.

Although detailed exemplary embodiments of the communication systemprovided in accordance with practice of the present invention aredisclosed herein, other suitable structures for practicing the presentinvention may be employed as will be apparent to persons of ordinaryskill in the art. Consequently, specific structural and functionaldetails disclosed herein are representative only; they merely describeexemplary embodiments of the invention.

Turning to FIG. 1, a communications network 100 in accordance with thepresent invention is shown. The network 100 includes a backbone network102 that is terminated by a Wireless Access Termination System (“WATS”)104. The WATS 104 includes an indoor unit (“WATS-IDU”) 106 that isconnected to an outdoor unit and antenna (“WATS-ODU”) 108. The WATSbroadcasts data to subscribers 110 on a downstream channel and thesubscribers send information to the WATS using a shared upstreamchannel.

In one embodiment of the network 100, the upstream channel has aspectral bandwidth of between 6 MHz and 15 MHz. The subscribers 110compete for access to the upstream channel. Therefore, the upstreamchannel is divided into time slots referred to as minislots. Eachminislot is allocated to a subscriber 110, which can transmitinformation on the upstream channel for the duration of that minislot.Minislots are allocated to subscribers 110 by the WATS 104. Subscribers110 communicate their bandwidth requirements to the WATS 104 and theWATS assigns minislots to the subscribers based on their bandwidthrequirements. The minislot assignments are communicated to thesubscribers 110 via broadcast on the downstream channel. In otherembodiments, the upstream channel can be further divided intosubchannels and each subchannel can have different length minislots.Preferably, the channel is divided into subchannels of one half or onefourth the total channel bandwidth.

Typically, the upstream channel is non-line of sight. Therefore, in onepreferred embodiment of the network 100, signals on the upstream channelare transmitted using Vector Orthogonal Frequency Division Multiplexing(“VOFDM”). VOFDM can be used because it is robust in the presence of thesevere multipath distortion generally present in high capacity, non-lineof sight wireless channels. Instead of sending all of the data on asingle very high speed channel that occupies the entire channelbandwidth, VOFDM involves separating the data into a number of separatestreams and then transmitting each stream on a separate carrier at amuch lower rate. Spectral efficiency is maximized by ensuring that thecarriers are spaced so that the signals transmitted on the carriers areorthogonal. The signals transmitted on the carriers are orthogonal whenthe amplitude of all of the signals at a particular frequency are zeroexcept for the signal being transmitted on that carrier frequency.

Estimation of the upstream channel is achieved by dividing the channelinto a total of N carriers or tones, including N_(data) data tones, vtraining tones and N_(zero) zero tones. The training tones enable theWATS 104 to estimate the characteristics of the upstream channel.Preferably, the training tones are spaced at intervals of N/v, with thefirst training tone placed at the lower frequency band edge. The zerotones are used to prevent the VOFDM signal interfering with adjacentchannels. Preferably, the zero tones are placed in the N_(zero)/2 leftmost and N_(zero)/2 right most tones that are not already designated astraining tones. The remaining tones are dedicated to the transmission ofdata. In one embodiment of the communications network 100, N is 512,N_(data) is 396, v is 64 and N_(zero) is 52. In other embodiments moreor less training and zero tones can be used.

The subscriber equipment is illustrated in greater detail in FIG. 2. Thesubscriber equipment includes a Wireless Modem (“WM”) 200 connected tocustomer premise equipment 202. The WM includes an indoor unit(“WM-IDU”) 204 that is connected to an outdoor unit and antenna(“WM-ODU”) 206. The WM-IDU comprises a controller 208 and memory 210,which are connected to a VOFDM transmitter 212 and a receiver 214. Boththe transmitter 212 and the receiver 214 have digital output and areconnected to an analog module 216, which converts the digital output toan analog radio frequency signal for transmission. The controller 208and memory 210 are connected to the customer premise equipment 202 andthe analog module 216 is connected to the WM-ODU 206. Preferably, theWM-ODU 206 comprises an antenna for transmitting and receiving radiofrequency signals. More preferably, the WM-ODU 206 comprises multipleantennas for improved reception of the VOFDM signal.

The WM 200 demodulates and decodes data broadcast by the WATS 104 on thedownstream channel. It also codes, modulates and transmits data from thesubscriber to the WATS 104 on the upstream channel.

The functions performed by the WM 200 in encoding and modulating datafor transmission are illustrated by the flow chart shown as FIG. 3. Theencoding and modulation process begins when the subscriber equipment 202generates a burst of data for transmission to the WATS 104. Thecontroller 208, then performs the step 302 of dividing the data intoblocks, where each block represents the amount of data to be transmittedin a single VOFDM burst.

The data blocks are encoded using an error correcting encoding on thetransmitter 212 in the step 304. The error correcting encoding enablesthe WATS 104 to correct some of the errors that can result duringtransmission. In one preferred embodiment Reed-Solomon encoding is used.Reed-Solomon encoding is a linear block Forward Error Correction (“FEC”)technique which increases the block size by R bytes. The Reed-Solomonencoding enables the WATS to correct up to R/2 byte errors in eachencoded data block. In other embodiments of the communications network,other FEC encoding techniques can be used.

The encoded data is then scrambled by the transmitter 212 in the step306, which reduces the probability of long sequences of ones or zerosbeing generated. Long sequences of ones or zeros can result in thepeak-to-mean power ratio of the transmitted signal being undesirablyhigh.

Following the scrambling, another encoding step 308 is performed by thetransmitter 212, which involves the use of a convolutional code. Aconvolutional code maps k bits of a continuous input stream on n outputbits. The convolutional encoding reduces the probability that bit errorswill occur. In one embodiment of the transmitter, the convolutionalencoder is composed of two components. The first is a standard length—7,rate—½ convolutional encoder, utilizing a standard pair of generatorpolynomials (171, 133). The second is a standard puncturing module withpatterns for producing coding rates of ⅔ and by deleting bits from theoutput of the rate ½ encoder. Each block is encoded independently,therefore, the data is fully flushed from the encoder between bursts.This is done by feeding six 0 bits into the encoder.

Following the convolutional encoding, the transmitter 212 performs thebit interleaving step 310. The bit interleaving ensures that narrowbandinterference which can corrupt several adjacent data tones does notdegrade the performance of communications on the upstream channel. Whenthe data transmitted on the affected data tones is de-interleaved at theWATS 104, the errors are spread throughout the data stream. Spreadingthe errors increases the likelihood that FEC encoding can be used tocorrect the errors. A large number of errors in a small number of datablocks can result in those data blocks being lost despite the FECencoding. However, if the same number of errors are spread over a largernumber of data blocks, then there is a greater likelihood that the FECencoding can be used to correct the errors so that none of the datablocks will be lost.

The interleaved bits are mapped to symbols in the step 312 by thetransmitter 212. The symbols chosen depend on the modulation schemebeing used by the WM 200. In one embodiment, the WM 200 is capable oftransmitting using Quadrature Phase Shift Keying (“QPSK”), 16 QuadratureAmplitude Modulation (“16-QAM”) or 64-QAM. In other embodiments, the WMcan be configured to provide a greater variety of modulation techniques.

Following the symbol mapping, the transmitter 212 inserts training andzero tones and then performs an Inverse Fast Fourier Transform (“IFFT”)314. The IFFT ensures that the transmitted signals are orthogonal. Thecomplex baseband OFDM signal is the inverse Fourier transform of the NQAM input symbols. An IFFT is used to reduce the number of calculationsrequired to generate the baseband VOFDM signal.

Following the IFFT, the transmitter 22 performs the step 316 of adding acyclic prefix to the IFFT output to ensure orthogonality of thetransmitted signals in the presence of a timing offset.

Finally, the step 318 of FIR filtering is performed. The FIR filteringfilters out reflections and provides spectral shaping to increasespectral efficiency. The transmitter 212 uses a frequency controlledoscillator to shift the complex filtered output to a desiredIntermediate Frequency (“IF”) and discards the imaginary portion of thesignal to provide a real, two-sided signal capable of being applied to adigital to analog converter in the analog module 216.

In one embodiment of the WM 200, shown in FIG. 4, the transmitter andreceiver are implemented using a BCM2200 integrated circuit 400manufactured by Broadcom Corporation of Irvine, Calif. The BCM2200 isconnected to the memory and controller by a bus 402. The controller isimplemented using a BCM3310 communications processor 404 manufactured byBroadcom Corporation. The BCM2200 is also connected to the analog module216. The analog module 216 is implemented using discrete components in amanner well known in the art and comprises a digital to analog converter406 for the upstream channel, an analog-to-digital converter 408 for thedownstream channel, a local voltage controlled oscillator 410 forshifting between the IF and the transmission frequency and filters 412for selecting the upstream or downstream channel signal and for removingreflections and distortions. Preferably, the upstream and downstreamchannel frequencies are located within the U.S. MMDS Band (2.500-2.686GHz) of the RF spectrum. Although, in other embodiments, the upstreamand downstream channel frequencies are located with the U.S. MDS Band(2.150-2.162 GHz), the unlicensed UNII Band or the European FixedWireless Band.

Referring again to FIG. 2, a closer inspection reveals that the WATS-IDU106 comprises a controller 250 and memory 252 connected to a transmitter254 and a VOFDM receiver 256. In addition, both the transmitter 254 andreceiver 256 are connected to an analog module 258. The controller isconnected to the backbone network 102 and the analog module is connectedto the WATS-ODU 108.

The WATS 104 demodulates and decodes data broadcast by the WM 200 on theupstream channel. It also codes, modulates and transmits data from thebackbone network 102 to the subscriber 110 on the downstream channel.

The performance of communications on a channel can be quantified usingcertain metrics such as Signal to Interference plus Noise Ratio(“SINR”), Bit Error Rate (“BER”), Codeword Error Rate (“CER”), SymbolError Rate (“SER”), number of corrected bit errors, number of correctedcodeword errors, the metrics within a Viterbi decoder, the Ricean Kfactor or other similar measurements. Typically, in a communicationsnetwork a set of minimum requirements is defined for transmission on achannel. These requirements establish values for some or all of theabove metrics that must be maintained at all times. In one preferredembodiment of the communications network 100, the only requirementspecified is that the CER must remain below 10⁻⁷. However, in otherembodiments different minimum requirements involving higher or lowertolerances or the use of different combinations of metrics can also berequired.

As previously discussed, the encoding and modulation scheme required tosatisfy the above minimum requirement varies depending on the quality ofthe channel where a particular subscriber 110 is located. In oneembodiment of the network 100, the symbol rate is fixed for eachchannel. However, individual subscribers 110 are able to transmit usingdifferent encoding schemes and symbol constellations.

In accordance with practice of the present invention, the encodingscheme and symbol constellation used by a subscriber is controlled bythe WATS 104. When the WATS 104 receives a signal from a subscriber 110,the WATS measures the quality of the signal and directs the subscriberto adopt a new encoding scheme and/or symbol constellation if thesubscriber is not optimally utilizing the channel. In this way,subscribers 110 that experience a high quality upstream channel are ableto achieve higher throughputs than subscribers 110 that experience a lowquality upstream channel and all subscribers 110 are able to satisfy theminimum requirements for transmission on the upstream channel.

FIG. 5A illustrates a process used by a WATS 104 to allocate an encodingscheme and symbol constellation to a subscriber 110. The WATS 104measures the quality of the received signal transmitted by thesubscriber 110 on the upstream channel in the step 502. The WATS 104then performs the step 504 of determining the optimal encoding schemeand symbol constellation for the transmission of signals on a channelwith the quality of the measured quality of the upstream channel. If theoptimal encoding scheme and symbol constellation is determined to bedifferent to the encoding scheme and symbol constellation being used bythe subscriber 110 in the step 506, then the WATS 104 instructs thesubscriber to change encoding scheme and/or symbol constellation to theoptimal encoding scheme and symbol constellation in the step 508. If theencoding scheme and symbol constellation being used by the subscriber110 is the same as the optimal encoding scheme and symbol constellation,then the WATS 104 allows the subscriber to continue to transmit usingthat encoding scheme and symbol constellation 510.

The steps 502 and 504 of FIG. 5A are shown in greater detail in FIG. 5B.FIG. 5B shows a process that can be used to determine the optimalencoding scheme and symbol constellation for transmission of signals ona channel with the quality of the measured quality of the upstreamchannel. The WATS performs the step 502 of measuring the quality of thereceived signal from the subscriber 110, then the WATS compares themeasured quality of the channel to a set of lower thresholds for the useof the encoding scheme and symbol constellation being utilized by thesubscriber in the step 550. If the thresholds are not satisfied then theWATS 104 performs the step 552 of selecting a more robust encodingscheme and/or a smaller symbol constellation and then repeating the step550. Comparing the measured signal quality to the lower threshold foruse of the selected encoding scheme and symbol constellation. When anencoding scheme and symbol constellation is found for which the measuredchannel quality satisfies lower thresholds of that encoding scheme andconstellation, then the WATS 104 compares the measured quality of thechannel to a set of upper thresholds for use of that encoding scheme andsymbol constellation in the step 554. If the thresholds are notsatisfied, then the WATS selects a less robust encoding scheme and/or alarger symbol constellation in the step 556 and then repeats the step554 comparing the measured signal quality to the upper threshold for useof the selected encoding scheme and symbol constellation. When anencoding scheme and symbol constellation is found for which the measuredquality of the channel satisfies the upper thresholds for that encodingscheme, then the WATS 104 performs the step 558 of selecting thatencoding scheme and symbol constellation as the optimal encoding schemeand symbol constellation.

Preferably, the communications network 100 of the present invention isable to predict degradation of the upstream channel and causesubscribers 110 to change encoding schemes and/or symbol constellationsprior to the degradation occurring. Predicting degradation enables thecommunications network 100 to respond before it fails to satisfy theminimum transmission requirements. Any of the metrics mentioned abovecan be used to measure channel quality, however, there are limitationsin their ability to predict channel degradation.

FIG. 6 illustrates the relationship between SINR and the codeword errorprobability for an Additive White Gaussian Noise (“AWGN”) channel, aRicean channel and a Rayleigh channel. The curve 600 illustrates therelationship between SINR and the codeword error probability for an AWGNchannel. The curve 602 illustrates the relationship between codeworderror probability for a Ricean channel and the curve 604 illustrates therelationship between codeword error probability for a Rayleigh channel.The line 606 indicates a codeword error probability of 10⁻⁷. A codeworderror probability that is less than 10⁻⁷ satisfies the minimum operatingrequirement for one embodiment of the wireless communications system100.

Assuming first that the channel is an AWGN channel, then the codeworderror probability experienced at the SINR indicated by the line 608satisfies the minimum transmission requirement. However, a smalldecrease in SINR results in the probability exceeding the minimumtransmission requirement. The absence of a gradual increase in CERrenders it difficult for the WATS to predict that the channel isdegrading using this metric alone. The same is also true of therelationship between BER and average SINR (not shown).

FIG. 6 also illustrates that measurement of SINR alone is unlikely toaccurately predict channel degradation because the threshold at whichthe channel degrades is dependent on the type of the upstream channel.When the SINR received at the WATS 104 is as indicated by the line 608,the minimum transmission requirement is met if the channel is an AWGNchannel. However, if the channel is either a Ricean channel or aRayleigh channel, then the minimum transmission requirement is not met.In addition, the average channel SINR is unlikely to predict a decreasein performance arising from strong narrowband interference.

The variance in the SINR or the SER are more useful for detectingnarrowband interference. Other metrics can be used for predictingdecreases in performance irrespective of the channel type includingcorrected bit errors, corrected codeword errors, Ricean K-factor and theViterbi path metrics.

Any of the above metrics can alone or in combination be used to measurethe quality of the upstream channel and to predict degradation in itsperformance. Preferably, the WATS 104 compares the SINR or SER tothreshold values in the decision steps 550 and 554 of FIG. 5B. Morepreferably, the WATS 14 compares the result of the following function tothe threshold values.

More preferably again, the WATS 104 compares a number of the abovemetrics to a set of predetermined thresholds and the WATS 104 requires achange in encoding scheme and/or symbol constellation if any of thethresholds are not satisfied.

The Elements of a controller 250 and receiver 256 used by a WATS 104 tomeasure channel quality and determine the encoding scheme that asubscriber 110 can use to make optimal use of its allocated transmissiontime is illustrated in FIG. 7. The receiver 256 comprises an upstreamburst receiver 702 connected to a decoding block 704 and an encodingscheme and symbol constellation assignment block 706. The decoding block704 is also connected to the encoding scheme and symbol constellationassignment block 706 and a MAC layer block 708.

In one embodiment of the WATS 104, a VOFDM burst signal is received atthe WATS-ODU 108 and is input to the upstream burst receiver 702. Theupstream burst receiver 702 demodulates the VOFDM burst signal and atthe same time, measures the SINR for each tone in the signal.

The upstream burst receiver 702 outputs the demodulated data to thedecoder block 704. The decoder block converts the symbols to bitrepresentations, then de-interleaves the received bits and performsconvolutional decoding. In one embodiment, the convolutional decoding isperformed using a digital Viterbi decoder. However, in other embodimentssequential decoders, analog Viterbi decoders or other decoders may beused.

Following the convolutional decoding, the bits are de-scrambled and thenthe FEC encoding is used to perform forward error correction. Inaddition to performing these functions, the decoder block 704 obtainsstatistics associated with the FEC such as the BER, CER, corrected biterrors and corrected codewords.

The decoded data then passes to the MAC layer block 708, whichreconstructs the packets and places the received data in memory.

The measurements of SINR, BER and CER provide inputs to the encodingscheme and symbol constellation assignment block 706, which uses thesestatistics to decide whether the subscriber 110 should change to a moreor less robust encoding scheme and/or a smaller or larger signalconstellation in accordance with the procedure outlined above.

If the encoding scheme and symbol constellation assignment block 706determines that the subscriber 110 can use a more efficient encodingscheme, then the encoding scheme and symbol constellation assignmentblock 706 generates a message to be sent to the subscriber 110instructing it to change encoding schemes. This message is thentransmitted by the WATS 104 on the downstream channel.

In one embodiment of the communications network 100, the efficiency ofthe process illustrated in FIG. 5 is increased by establishing a set ofpredetermined encoding scheme and symbol constellation configurations.Each of the predetermined encoding scheme and symbol constellationconfigurations is designed to satisfy the minimum requirements andprovide high throughput for a given range of channel qualities. The WATS104 can then direct the subscriber 110 to adopt the predeterminedencoding scheme and symbol constellation configuration most appropriatefor use at the measured channel quality. The predetermined encodingscheme and symbol constellation configurations used in one preferredembodiment of the communications network 100 are shown in TABLE 2.

TABLE 2 Encoding scheme and symbol constellation configurations Comb. IDN ν N_(zero) N_(data) Sym. Const. R Conv. I 128 16 22 90 64-QAM 14 0.833II 128 16 22 90 64-QAM 14 0.667 III 128 16 22 90 64-QAM 10 0.5 IV 128 1622 90 16-QAM 10 0.5 V 128 16 22 90 QPSK 10 0.667Where Comb. ID is the encoding scheme and symbol constellationidentifier; N is the number of VOFDM tones; v is the number of trainingtones; N_(zero) is the number of zero tones; N_(data) is the number ofdata tones; Symb. Const, is the symbol constellation being used; R isthe number bytes added by the Reed-Solomon encoding; and Conv. is therate of the convolutional encoder.

In one embodiment of the communications network 100, the thresholds atwhich these different encoding scheme and symbol constellationconfigurations are used are determined automatically by the WATS 104using known test transmissions from the subscriber 110.

In another embodiment, a set of thresholds are defined for the minimumSINR required for the encoding scheme and signal constellationconfiguration to be used. These thresholds are shown in TABLE 3.

TABLE 3 Minimum SINR thresholds Comb. ID SINR threshold I >21 dB II >16dB III >12 dB IV  >9 dB V  >4 dB

In other embodiments, thresholds are defined for a number of performancemetrics in combination or separately.

In one embodiment of the WATS 104, the transmitter 254 and receiver 256are implemented using a BCM92210 linecard manufactured by BroadcomCorporation and the controller 250 is implemented using a Pentiummicroprocessor manufactured by Intel Corporation of Santa Clara, Calif.In other embodiments other transmitters, VOFDM receivers and controllersor microprocessors can be used.

Turning now to FIG. 8, a process 800 for allocating an encoding schemeand symbol constellation to a new subscriber 110 is illustrated.Initially 802, the subscriber 110 attempts to establish communicationsor register with the WATS 104 by sending a registration request on theupstream channel. The subscriber 110 attempts to register with the WATS104 using the most robust of the predetermined encoding scheme andsymbol constellation configurations. The WATS 104 receives theregistration request transmission and then performs the step 804 ofdetermining if the subscriber can support the minimum requirements fortransmission. The WATS 104 completes this step by measuring thecharacteristics of the received transmission and determining whetherthey satisfy the minimum requirements. If the minimum requirements arenot met, the WATS 104 refuses the registration request 806. A refusalmust be made because the subscriber WM 200 cannot transmit using a morerobust encoding scheme. Therefore, the subscriber 110 cannot alter theencoding or modulation of the transmission in any way that will causethe minimum requirements to be met.

If the transmission satisfies the minimum requirements, then the WATS104 performs the step 808 of assigning a predetermined encoding schemeand symbol constellation configurations to the subscriber 110. Once thispredetermined encoding scheme and symbol constellation configurationassignment has been completed, future transmissions by the subscriber110 must use this encoding scheme unless the WATS 104 directs otherwise.The WATS 104 continually monitors the quality of subscribertransmissions 810 and instructs the subscribers 110 to change encodingschemes and/or symbol constellations as required.

A process 900 used in one embodiment of the communications network 100to change the encoding scheme and symbol constellation sizes being usedby a WM 200 is illustrated in FIG. 9. First 902, the encoding scheme andsymbol constellation assignment block 706 determines that a change ofencoding scheme and/or symbol constellation is required. The WATS 104then performs the step 904 of transmitting an Encoding and Symbolconstellation Change Request (“ESC-Req”) to the WM 200. In step 906, theWM 200 receives the ESC-Req. Once the ESC-Req is received, the WM 200then performs the step 908 of transmitting an Encoding and Symbolconstellation Change Response (“ESC-Rsp”). The WATS 104 receives theESC-Rsp in step 910 and transmits a Encoding and Symbol constellationChange Acknowledgment (“ESC-Ack”) in step 912. Once the WM 200 receivesthe ESC-Ack in step 914, it is then able to perform the step 916 ofadopting the new encoding scheme. Once this step is complete, all futuretransmissions from that subscriber 110 must use the new encoding schemeand symbol constellation unless the WATS 104 specifies otherwise.

The delay between the decision by the WATS 104 that a subscriber 110must adopt a new encoding scheme and symbol constellation and theadoption by the subscriber of the new encoding scheme and symbolconstellation is illustrated in FIG. 10. The delay is equal to the sumof the ESC-Req transmission time 1000, the WM processing time 1002, theESC-Rsp transmission time 1004, the WATS processing time 1006 and theESC-Ack transmission time 1008. The subscriber 110 is unable to transmitdata during the period from when the ESC-Req is received by the WM 200until when the ESC-Ack is received by the WM. Therefore, frequentchanges of encoding scheme and/or symbol constellation can reduce WM 200throughput.

In one embodiment of the communications network 100, the predeterminedencoding scheme and symbol constellation configurations are chosen sothat the range in channel qualities that the encoding scheme and symbolconstellation configurations are designed to be utilized within overlapwith each other. This overlapping provides hysteresis, which reduces thefrequency with which a subscriber 110 must alter encoding scheme andsymbol constellations. Reducing the frequency of changing encodingscheme and/or symbol constellation eliminates the communication overheadassociated with these changes and increases throughput by enabling thesubscriber 110 to spend more time transmitting data.

The encoding scheme and constellation configurations and the ranges ofSINRs in which these configurations can be used for one embodiment ofthe communications network 100 that incorporates hysteris are shown inTABLE 4.

TABLE 4 SINR ranges for use of encoding scheme and constellation sizeconfigurations Comb. ID Min. SINR Max. SINR I 21 dB N/A II 16 dB 22 dBIII 12 dB 17 dB IV  9 dB 13 dB V  4 dB 10 dB

The communications network 100 of the present invention transmitsdifferent types of traffic, such as voice, data or video traffic. Thesedifferent types of traffic generally have different Quality of Service(“QoS”) requirements. The QoS required by a type of traffic is oftendetermined by the nature of the traffic. Voice traffic has lowcommunication bandwidth requirements but is intolerant to delays orinformation arriving out of order. The same is also true of videotraffic, but video traffic also requires much higher bandwidths. Datatransfer has different characteristics. Data transfer usually occurs inbursts involving periods where little bandwidth is required. Delay andorder are largely irrelevant in data transfers, the primary requirementis often speed.

In one embodiment of the communications network 100, the customerpremise equipment 202 generates traffic streams and specifies the QoSrequired for transmission of each traffic stream. Consequently, thenetwork 100 is configured such that the traffic stream is transmitted onthe upstream channel if there are sufficient minislots available toguarantee that the QoS requirements will be met.

A process 1100 used by communications network 100 to determine if theupstream channel can meet the QoS requirements of a new traffic streamis illustrated in FIG. 11. Firstly, the customer premise equipmentgenerates a traffic stream in the step 1102. Prior to commencingtransmission of the traffic stream, the WM 200 performs the step 1104 ofsending a message to the WATS 104 requesting that the WM 200 be allowedto transmit a new traffic stream on the upstream channel. The messagealso contains information concerning the QoS required by the new trafficstream. The WATS 104 then is faced with the decision 1106 of determiningif it can allocate sufficient minislots to the WM 200 to satisfy therequested QoS requirements.

If the WATS 104 is unable to allocate enough minislots, then it performsthe step 1108 of sending a message to the WM 200. The messagecommunicates the WATS refusal to accept transmission of the new trafficstream on the upstream channel. If sufficient minislots are available tosatisfy the requested QoS requirements, then the WATS 104 completessteps 1110 and 1112 by sending a message to the WM 200 accepting the newtraffic stream and then allocating the required minislots to the WM 200.

The communications network 100 of the present invention supportsadaptive encoding and modulation on the upstream channel. Changes in theencoding scheme and/or symbol constellation used by a WM 200 can effectthe QoS experienced by the traffic streams that the WM is transmitting.If the WM 200 changes to a more robust encoding scheme and/or a smallersymbol constellation, then its effective data rate on the upstreamchannel is reduced. Conversely, if the WM 200 changes to a less robustencoding scheme and/or a larger symbol constellation then, the effectivedata rate of the WM 200 on the upstream channel increases. These changesin effective data rate can effect the QoS experienced by the trafficstreams being transmitted by the WM 200. Traffic streams that demand aconstant data rate are particularly affected by changes which reduce theeffective data rate of the WM 200. If the effective data rate of a WM200 is reduced by a change in encoding scheme and/or symbolconstellation, the WATS 104 can allocate additional minislots to the WMto ensure the QoS requirements of its traffic streams are met. Inaddition, the WATS 104 can reallocate some of the minislots allocated tothat WM 104, when its effective data rate is increased. The minislotscan only be reallocated if doing so does not jeopardize the QoS of theWM 200 traffic streams.

Preferably, each traffic stream is assigned a priority. Therefore, ifthere are insufficient unallocated minislots to guarantee the QoS of alltraffic streams, then the WATS 104 can reallocate minislots from trafficstreams with low priorities to traffic streams with higher priorities.

An embodiment of a controller 250′ and receiver 256′ in accordance withpractice of the present invention are illustrated in FIG. 12. Thereceiver 256′ is similar to the receiver 256 of FIG. 7 and thecontroller 250′ is similar to the controller 250 of FIG. 7, with theaddition of a minislot scheduler 1200 that is connected to the encodingscheme and symbol constellation assignment block 706. The minislotscheduler 1200 maintains a schedule of the minislots assigned todifferent subscriber traffic streams and the priorities of thosestreams. The minislot scheduler 1200 also allocates minislots to the WMs200 and reallocates scheduled minislots, when the upstream channel iscongested.

A process 1300 used by the controller 250′ for allocating minislots inresponse to changes in the encoding scheme and/or symbol constellationused by a WM 200 is illustrated in FIG. 13. When the encoding scheme andsymbol constellation assignment block 706 determines that a change inencoding scheme is required 1302, it communicates its decision to the WM200 and the minislot scheduler 1200 in the step 1304. The minislotscheduler 1200 then performs the step 1306 of determining whether the WM200 has been allocated sufficient minislots to meet the QoS requirementsof the traffic streams the WM is transmitting at that time.

If the WM 200 has been allocated sufficient minislots to meet the QoSrequirements of the traffic streams it is transmitting, then theminislot scheduler 1200 must make a decision 1308. This decisionrequires the minislot scheduler 1200 to determine if the currentminislot allocation of the WM 200 is in excess of the number ofminislots required to meet the QoS requirements of the traffic streamsbeing transmitted by the WM 200. If the current minislot allocation isnot in excess of the number of minislots required to meet the QoSrequirements of the traffic streams being transmitted by the WM 200,then the minislot scheduler 1200 does nothing 1310. If the WM 200 hasbeen allocated an excess of minislots, then the minislot scheduler 1200causes a message to be sent to the WM informing it that the excessminislots are no longer assigned to it. The minislot scheduler 1200 isthen free to allocate these minislots to other traffic streams asrequired.

If the WM 200 has not been allocated with sufficient minislots to meetthe QoS requirements of the traffic streams it is transmitting, then theminislot scheduler makes the decision 1314 whether there are enoughunallocated minislots to meet the requirements of the WM following thechange in encoding scheme. If there are sufficient unallocatedminislots, then the minislot scheduler 1200 allocates the minislots tothe WM 200 and causes a message to be sent to the WM informing it of thenew minislot allocation.

If there are insufficient unallocated minislots to meet the QoSrequirements of the traffic streams being transmitted by the WM 200,then the minislot allocation system determines in the step 1318 if theminislot requirements of the WM can be met by the reallocation ofpreviously allocated minislots. For the minislot requirements of the WM200 to be met in this way, the traffic streams being transmitted by theWM must have higher priority than other traffic streams on the upstreamchannel. If this is the case, then the required minislots are assignedto the WM 200 in the step 1320 by reallocating minislots assigned tolower priority traffic streams.

If there are insufficient minislots allocated to lower priority trafficstreams to meet the requirements of the WM 200, then the minislotscheduler causes a message to be sent to the WM informing it that thereare insufficient minislots available. The WM 200 must then perform thestep 1322 of negotiating new QoS requirements for its traffic streamswith the customer premise equipment 202.

While the above description contains many specific features of theinvention, these should not be construed as limitations on the scope ofthe invention, but rather as an example of one preferred embodimentthereof. Many other variations are possible. Accordingly, the scope ofthe invention should be determined not by the embodiments illustrated,but by the appended claims and their legal equivalents.

CONCLUSION

It is to be appreciated that the Detailed Description section, and notthe Abstract section, is intended to be used to interpret the claims.The Abstract section may set forth one or more, but not all exemplaryembodiments, of the present invention, and thus, are not intended tolimit the present invention and the appended claims in any way.

The present invention has been described above with the aid offunctional building blocks illustrating the implementation of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries may be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

It will be apparent to those skilled in the relevant art(s) that variouschanges in form and detail can be made therein without departing fromthe spirit and scope of the invention. Thus, the present inventionshould not be limited by any of the above-described exemplaryembodiments, but should be defined only in accordance with the followingclaims and their equivalents.

1. A method for selecting one of a plurality of sets of encoding schemesand symbol constellations, each set including an encoding scheme and acorresponding symbol constellation, for use by a subscriber, comprising:(a) measuring a quality of a received signal transmitted by thesubscriber, the received signal being encoded using a set from among theplurality of sets of encoding schemes and symbol constellations; (b)determining an optimal set from among the plurality of sets of encodingschemes and symbol constellations based upon the quality of the receivedsignal; (c) instructing the subscriber to change its encoding scheme toan encoding scheme corresponding to the optimal set when its encodingscheme differs from the encoding scheme corresponding to the optimalset; and (d) instructing the subscriber to change its symbolconstellation to a symbol constellation corresponding to the optimal setwhen its symbol constellation differs from the symbol constellationcorresponding to the optimal set.
 2. The method of claim 1, wherein step(a) comprises: (a)(i) measuring at least one of a group consisting of: asignal to interference plus noise ratio (SINR); a SINR variance; a biterror rate (BER); a codeword error rate (CER); a symbol error rate(SER); a channel power; a number of corrected bit errors; a number ofcorrected codeword errors; a Ricean K-factor; and a Viterbi decoder pathmetric of the received signal transmitted by the subscriber.
 3. Themethod of claim 1, wherein the received signal is further encoded usinga fixed symbol rate.
 4. The method of claim 1, wherein each of the setsof encoding schemes and symbol constellations is configured to operatein a range of channel qualities, wherein more than one of the ranges ofchannel qualities is configured to overlap each other.
 5. The method ofclaim 1, wherein at least one of the sets of encoding schemes and symbolconstellations utilizes as its respective encoding scheme one of a groupconsisting of: Reed-Solomon encoding configured to correct a number ofbyte errors per codeword; and a convolutional encoding of a rate.
 6. Themethod of claim 1, wherein at least one of the sets of encoding schemesand symbol constellations utilizes as its respective symbolconstellation one of a group consisting of: Quadrature Phase ShiftKeying (QPSK); 16 Quadrature Amplitude Modulation (16-QAM); and 64Quadrature Amplitude Modulation (64-QAM).
 7. The method of claim 1,wherein step (b) comprises: (b)(i) comparing the quality of the receivedsignal to one or more lower thresholds; (b)(ii) selecting a second setfrom among the plurality of sets of encoding schemes and symbolconstellations; (b)(iii) repeating steps (b)(i) and (b)(ii) by selectingsets from among the plurality of sets of encoding schemes and symbolconstellations until the one or more lower thresholds are satisfied;(b)(iv) comparing the quality of the received signal to one or moreupper thresholds; (b)(v) repeating steps (b)(i) through (b)(iv) untilthe one or more upper thresholds are satisfied by selecting sets fromamong the plurality of sets of encoding schemes and symbolconstellations until the one or more lower thresholds and the one ormore upper thresholds are satisfied; and (b)(vi) designating the secondset that satisfied the one or more lower thresholds and the one or moreupper thresholds as the optimal set.
 8. The method of claim 7, whereinstep (b)(iii) comprises: (b)(iii)(A) repeating steps (b)(i) and (b)(ii)by selecting the sets from among the plurality of sets of encodingschemes and symbol constellations that may be characterized as having atleast one of a group consisting of: a more robust encoding scheme, and asmaller symbol constellation.
 9. The method of claim 7, wherein step(b)(v) comprises: (b)(v)(A) repeating steps (b)(i) through (b)(iv) untilthe one or more upper thresholds are satisfied by selecting sets fromamong the plurality of sets of encoding schemes and symbolconstellations that may be characterized as having at least one of agroup consisting of: a lesser robust encoding scheme, and a largersymbol constellation.
 10. The method of claim 1, wherein step (c)comprises: (c)(i) sending an Encoding and Symbol constellation ChangeRequest (ESC-Req) to the subscriber when the encoding scheme of thesubscriber differs from the encoding scheme corresponding to the optimalset; (c)(ii) receiving a Encoding and Symbol constellation ChangeResponse (ESC-Rsp) from the subscriber in response to the ESC-Req; and(c)(iii) sending an Encoding and Symbol constellation ChangeAcknowledgement (ESC-Ack) to the subscriber in response to the ESC-Rspto cause the subscriber to change its encoding scheme to the encodingscheme corresponding to the optimal set.
 11. The method of claim 1,wherein step (d) comprises: (d)(i) sending an Encoding and Symbolconstellation Change Request (ESC-Req) to the subscriber when the symbolconstellation of the subscriber differs from the symbol constellationcorresponding to the optimal set; (d)(ii) receiving a Encoding andSymbol constellation Change Response (ESC-Rsp) from the subscriber inresponse to the ESC-Req; and (d)(iii) sending an Encoding and Symbolconstellation Change Acknowledgement (ESC-Ack) to the subscriber inresponse to the ESC-Rsp to cause the subscriber to change its symbolconstellation to the symbol constellation corresponding to the optimalset.
 12. An apparatus for selecting one of a plurality of sets ofencoding schemes and symbol constellations, each set including anencoding scheme and a corresponding symbol constellation, for use by asubscriber, comprising: an upstream burst receiver configured to measurea quality of a received signal transmitted by the subscriber, thereceived signal being encoded using a set from among the plurality ofsets of encoding schemes and symbol constellations; and an encodingscheme and symbol constellation assignment block configured; (i) todetermine an optimal set from among the plurality of sets of encodingschemes and symbol constellations based upon the quality of the receivedsignal, (ii) to instruct the subscriber to change its encoding scheme toan encoding scheme corresponding to the optimal set when its encodingscheme differs from the encoding scheme corresponding to the optimalset, and (iii) to instruct the subscriber to change its symbolconstellation to a symbol constellation corresponding to the optimal setwhen its symbol constellation differs from the symbol constellationcorresponding to the optimal set.
 13. The apparatus of claim 12, whereinthe quality of the received signal is at least one of a group consistingof: a signal to interference plus noise ratio (SINR); a SINR variance; abit error rate (BER); a codeword error rate (CER); a symbol error rate(SER); a channel power; a number of corrected bit errors; a number ofcorrected codeword errors; a Ricean K-factor; and a Viterbi decoder pathmetric of the received signal transmitted by the subscriber.
 14. Theapparatus of claim 12, wherein the received signal is further encodedusing a fixed symbol rate.
 15. The apparatus of claim 12, wherein eachof the sets of encoding schemes and symbol constellations is configuredto operate in a range of channel qualities, wherein more than one of theranges of channel qualities is configured to overlap each other.
 16. Theapparatus of claim 12, wherein at least one of the sets of encodingschemes and symbol constellations utilizes as its respective encodingscheme one of a group consisting of: Reed-Solomon encoding configured tocorrect a number of byte errors per codeword; and a convolutionalencoding of a rate.
 17. The apparatus of claim 12, wherein at least oneof the sets of encoding schemes and symbol constellations utilizes asits respective symbol constellation one of a group consisting of:Quadrature Phase Shift Keying (QPSK); 16 Quadrature Amplitude Modulation(16-QAM); and 64 Quadrature Amplitude Modulation (64-QAM).
 18. Theapparatus of claim 12, wherein the encoding scheme and symbolconstellation assignment block is configured to determine the optimalset by: (i) comparing the quality of the received signal to one or morelower thresholds; (ii) selecting a second set from among the pluralityof sets of encoding schemes and symbol constellations; (iii) repeatingsteps (b)(i) and (b)(ii) by selecting sets from among the plurality ofsets of encoding schemes and symbol constellations until the one or morelower thresholds are satisfied; (iv) comparing the quality of thereceived signal to one or more upper thresholds; (v) repeating steps(b)(i) through (b)(iv) until the one or more upper thresholds aresatisfied by selecting sets from among the plurality of sets of encodingschemes and symbol constellations until the one or more lower thresholdsand the one or more upper thresholds are satisfied; and (vi) designatingthe second set that satisfied the one or more lower thresholds and theone or more upper thresholds as the optimal set.
 19. The apparatus ofclaim 18, wherein the encoding scheme and symbol constellationassignment block is configured to select sets from among the pluralityof sets of encoding schemes and symbol constellations that may becharacterized as having at least one of a group consisting of: a morerobust encoding scheme, and a smaller symbol constellation.
 20. Theapparatus of claim 18, wherein the encoding scheme and symbolconstellation assignment block is configured to select sets from amongthe plurality of sets of encoding schemes and symbol constellations thatmay be characterized as having at least one of a group consisting of: alesser robust encoding scheme, and a larger symbol constellation. 21.The apparatus of claim 12, wherein the encoding scheme and symbolconstellation assignment block is further configured to send an Encodingand Symbol constellation Change Request (ESC-Req) to the subscriber whenthe encoding scheme of the subscriber differs from the encoding schemecorresponding to the optimal set, to receive a Encoding and Symbolconstellation Change Response (ESC-Rsp) from the subscriber in responseto the ESC-Req, and to send an Encoding and Symbol constellation ChangeAcknowledgement (ESC-Ack) to the subscriber in response to the ESC-Rspto cause the subscriber to change its encoding scheme to the encodingscheme corresponding to the optimal set.
 22. The apparatus of claim 12,wherein the encoding scheme and symbol constellation assignment block isconfigured to send an Encoding and Symbol constellation Change Request(ESC-Req) to the subscriber when the symbol constellation of thesubscriber differs from the symbol constellation corresponding to theoptimal set, to receive a Encoding and Symbol constellation ChangeResponse (ESC-Rsp) from the subscriber in response to the ESC-Req, andto send an Encoding and Symbol constellation Change Acknowledgement(ESC-Ack) to the subscriber in response to the ESC-Rsp to cause thesubscriber to change its symbol constellation to the symbolconstellation corresponding to the optimal set.